1. Field of the Invention
The invention pertains to the field of variable camshaft timing mechanisms. More particularly, the invention pertains to a hydraulic variable camshaft timing mechanism with a default mode.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the relative timing between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more “vane phasers” on the engine camshaft (or camshafts, in a multiple-camshaft engine). As shown in the figures, vane phasers have a rotor 105 with one or more vanes 104, mounted to the end of the camshaft 126, surrounded by a housing assembly 100 with the vane chambers into which the vanes fit. It is possible to have the vanes 104 mounted to the housing assembly 100, and the chambers in the rotor assembly 105, as well. The housing's outer circumference 101 forms the sprocket, pulley or gear accepting drive force through a chain, belt, or gears, usually from the crankshaft, or possible from another camshaft in a multiple-cam engine.
Apart from the camshaft torque actuated (CTA) variable camshaft timing (VCT) systems, the majority of hydraulic VCT systems operate under two principles, oil pressure actuation (OPA) or torsional assist (TA). In the oil pressure actuated VCT systems, an oil control valve (OCV) directs engine oil pressure to one working chamber in the VCT phaser while simultaneously venting the opposing working chamber defined by the housing, the rotor, and the vane. This creates a pressure differential across one or more of the vanes to hydraulically push the VCT phaser in one direction or the other. Neutralizing or moving the valve to a null position puts equal pressure on opposite sides of the vane and holds the phaser in any intermediate position. If the phaser is moving in a direction such that valves will open or close sooner, the phaser is said to be advancing and if the phaser is moving in a direction such that valves will open or close later, the phaser is said to be retarding.
The torsional assist (TA) systems operates under a similar principle with the exception that it has one or more check valves to prevent the VCT phaser from moving in a direction opposite than being commanded, should it incur an opposing force such as torque.
The problem with OPA or TA systems is that the oil control valve defaults to a position that exhausts all the oil from either the advance or retard working chambers and fills the opposing chamber. In this mode, the phaser defaults to moving in one direction to an extreme stop where the lock pin engages. The OPA or TA systems are unable to direct the VCT phaser to any other position during the engine start cycle when the engine is not developing any oil pressure. This limits the phaser to being able to move in one direction only in the default mode. In the past this was acceptable because at engine shut down and during engine start the VCT phaser would be commanded to lock at one of the extreme travel limits (either full advance or full retard). However, recent calibration work has demonstrated that there is considerable benefit to start the engine with the VCT system in some intermediate position and not at the extreme stops.
In addition with large range of angular motion on a VCT phaser it is necessary to start in the mid travel position because the extreme stops may be positions where the engine will not start or be damaged during an engine start. It is equally desirable that if the engine is shut down and the phaser is not in the optimum mid start position that the phaser is able to move in either direction during engine cranking to get back to the optimum start position somewhere in mid travel. Current OPA and TA VCT systems are not capable of such recovery because by default they exhaust the oil from one of the working chambers (either advance or retard) and therefore render that chamber a non working chamber at shut down, such that the VCT phaser can only move in one direction.